The purpose of this study is to delay the onset of clinically definite multiple sclerosis (CDMS) in subjects who have experienced a first and recent onset of a demeyelinating event involving the optic nerve, spinal cord, or brainstem, and who are at high risk for multiple sclerosis (MS). The primary endpoint of the study will be the time to developement of CDMS as determined by the onset of a new neurological event, i.e., new neurological symptoms lasting more than 48 hours in a subject who had been neurologically stable or improving at least 30 days following initiation of study drug treatment. Secondary endpoints include change in T2 lesion volume from baseline to Year 3, number and volume of gadolinium-enhanced lesions at Year 3, and number of new or enlarged T2 hyperintense lesions at Year 3. One interm efficacy analysis will be performed after all subjects have completed 2 years of treatment. The study is sized under the assumption that 50% of subjects treated with placebo will reach CDMS by 3 years and that treatment with AVONEX tm will reduce this percentage to 33%. Interm safety analyses will be performed at intervals throughout the study.